1. Field of the Invention
The present invention relates to an amplifier having a driver stage and an output stage connected in cascade.
2. Discussion of the Related Art
"Buffer" or separation amplifiers, that is, amplifiers in which the input circuit is fairly insensitive to variations in the load impedance, are used in many circuit systems. A buffer amplifier is consequently used when a signal generated by a circuit with high output impedance has to control a load with low impedance, such as a capacitor with a large capacitance, if the control signal is of high frequency, or a resistor with low resistance.
A simple known buffer amplifier is shown in FIG. 1 of the appended drawings. It is constituted by two complementary bipolar transistors one of npn type, indicated Q1, and one of pnp type, indicated Q2, connected in a push-pull arrangement between the two poles, that is, the positive pole, indicated vdd, and the negative pole, indicated gnd, of a voltage supply. The bases of the two transistors are joined together to form the input terminal IN of the amplifier and the connection point between the two emitters is the output terminal OUT of the amplifier. A circuit with a high output impedance connected to the input IN of the amplifier is represented by a high-frequency alternating-voltage generator vi and by a resistor Ri in series, and a low impedance load is represented by a capacitor C.
In operation, the two transistors behave alternately like an amplifier with a common collector (an emitter follower) during the positive half-wave and during the negative half-wave of the input signal, respectively, so that the circuit has all of the characteristics of an amplifier of this type, amongst which is a low output impedance.
The known circuit described above has an "offset", that is, a deviation between the output voltage level and the input voltage level, which is substantially equal to the voltage drop at the forward biased base-emitter junction of the npn transistor or of the pnp transistor. The offset causes a distortion of the signal, known as "cross-over distortion" in the case of sinusoidal input signals, which is more notable the smaller the input signal. Moreover, the voltage drop at the forward biased base-emitter junction varies from one specimen of the same integrated circuit in which the "buffer" is formed to another, because of the inevitable variations in the processing parameters, and also varies with the operating temperature.
There are applications in which the supply voltage is low, so that the signals to be processed are also of low level, or in which the variability of the offset creates serious problems in the design of the circuits downstream of the amplifier.
An object of the present invention is therefore to propose an amplifier of the type having a driver stage and an output stage connected in cascade which has a very low or even zero offset.